According to a study by researchers at Kansas University, it's probably because you're religious. They discovered that atheists had much better sex lives than people who followed a religion.

The survey, called 'Sex and Secularism' looked at the sex lives of 14,500 people. The survey discovered that all the people they talked to began having sex at a similar age and had the same amount of sex per week.

However religious people reported feeling guilty after sex and rated the quality of their sex lives far lower than non-believers.

Mormons came top of the list for feeling guilty about sex, with an average score of 8.19 out of 10, followed by Jehovah's Witnesses, and members of the Pentecostal, Seventh Day Adventist, and Baptist churches.

The survey showed Catholics had an average guilt level of 6.34, but non-believers scored lowest, with atheists at 4.71 and agnostics at 4.81.

The study showed 79.9% of people brought up in devoutly religious homes reported feeling guilty about sexual fantasies or actions, compared to only around a quarter of those brought up in non-religious homes.

Psychologist Darrel Ray said: "Our data shows that people feel very guilty about their sexual behaviour when they are religious, but that does not stop them: it just makes them feel bad."